C ^84, 3
XIV. Experiments on the Refrangibility of the invisible Rays of the
Sun. By William Herschel, LUD. F. R. S.
Read April 2^4, 1800.
In that section of my former paper which treats of radiant heat,
it was hinted, though from imperfect experiments, that the range
of its refrangibility is probably more extensive than that of the
prismatic colours ; but, having lately had some favourable sun-
shine, and obtained a sufficient confirmation of the same, it will
be proper to add the following experiments to those which have
I provided a small stand, with four short legs, and covered it
with white paper.* On this I drew five lines, parallel to one end
of the stand, at half an inch distance from each other, but so
that the first of the lines might only be j; of an inch from the
edge. These lines I intersected at right angles with three others ;
the 2d and 3d whereof were, respectively, at 2|- and at 4 inches
from the first.
The same thermometer^ that have before been marked
No. 1, s, and 3, mounted upon their small inclined planes, were
then placed so as to have the centres of the shadow of their
balls thrown on the intersection of these lines. Now, setting
my little stand upon a table, I caused the prismatic spectrum to
fall with its extreme colour upon the edge of the paper, so that
none might advance beyond the first line. In this arrangement,
♦ See Plate XL
Dr. Herscwei's Experiments, &c, 285
all the spectrum, except the vanishing last quarter of an inch,
which served as a direction, passed down by the edge of the
stand, and could not interfere with the experiments. I had also
now used the precaution of darkening the window in which the
prism was placed, by fixing up a thick dark green curtain, to
keep out as much light as convenient.
The thermometers being perfectly settled at the temperature
of the room, I placed the stand so that part of the red colour,
refracted by the prism, fell on the edge of the paper, before the
thermometer No. 1, and about half way, or i-^ inch, towards
the second : it consequently did not come before that, or the
3d thermometer, both which were to be my standards. During
the experiment, I kept the last termination of visible red
carefully to the first line, as a limit assigned to it, by gently
moving the stand when required ; and found the thermometers,
which were all placed on the second linp, affected as follows.
Here the thermometer No. 1 rose 6| degrees, in 10 minutes^
when its centre was placed ^ inch beyond visible light.
In order to have a confirmation of this fact, I cooled the
thermometer No. 1, and placed No. s in the room of it : I also
put No. 3 in the plaee of No. 2, and No. 1 in that of No. 3 ;
and, having exposed them as before, arraja^ed on the second
line, I had the following result.
386 Dr. Herschel's Experiments on the Refrangihility
Here the thermometer No. 2 rose 2^ degrees, in 13 minutes;
and being, as has been noticed before, much more sensible than
No. 1, it came to the temperature of its situation in a short time;
but I left it exposed longer, on purpose to be perfectly assured
of the result. Its shewing but 2| degrees advance, when No. 1
shewed 6|-, has also been accounted for before.
It being now evident that there was a refraction of rays coming
from the sun, which, though not fit for vision, were yet highly
invested with a power of occasioning heat, I proceeded to ex-
amine its extent as follows.
The thermometers were arranged on the third line, instead
of the second; and the stand was, as before, immersed up to the
first, in the coloured margin of the vanishing red rays. The
result was thus-
No. 1. No. 2. No. 3.
- . 46I
Here the thermometer No. 1 rose 5^ degrees, in 13 minutes^
at 1 inch behind the visible light of the red rays.
I placed now the thermometers on the 4th line, instead of
the 3d ; and, proceeding as before, I had the following result.
of the invisible Rays of the Sun. £87
No. 1. No. 2. No. 3.
48i - - 48i - - 471
5ii - - 48I - - 471-
Therefore, the thermometer No. 1 rose 3- degree, in 10
minutes, at 1^ inch beyond the visible light of the red rays.
I might now have gone on to the 5th line ; but, so fine a day,
with regard to clearness of sky and perfect calmness, was not
to be expected often, at this time of the year; I therefore
hastened to make a trial of the other extreme of the prismatic
spectrum. This was attended with some difficulty, as the illu-
mination of the violet rays is so feeble, that a precise termination
of it cannot be perceived. However, as well as could be judged,
I placed the thermometers one inch beyond the reach of the
violet rays, and found the ^result as follows.
►.1* No. 2. No. 3.
Here the several indications of the thermometers, two of
which. No. 1 and 2, were used as variable, while the 3d was
kept as the standard, were read off during a time that lasted
13 minutes ; but they aflbrd, as may be seen by inspection, no
ground for ascribing any of their small changes to other causes
than the accidental disturbance which will arise from the motion
of the air, in a room where some employment is carried on.
I exposed the thermometer now to the line of the very first
perceptible violet light; but so that No. 1 and > might again
MDCCC, P p
£88 Dr:lrLERscuEL's Experiments on the Refrangibility
be in the illumination, while No, 3 remained a standard. The
result proved as follows.
No. 1. No. 2, No. 3.
Here the thermometer No. 1 rose 1 degree, in 15 minutes;
and No. 2 rose |- degree, in the same time.
From these last experiments, I was now sufficiently persuaded,
that no rays which might fall beyond the violet, could have
any perceptible power, either of illuminating or of heating ; and
that both these powers continued together throughout the pris-
matic spectrum, and ended where the faintest violet vanishes.
A very material point remained still to be determined, which
was, the situation of the maximum of the heating power.
As I knew already that it did not lie on the violet side of the
red, I began at the full red colour, and exposed my thermometers,
arranged on a line, so as to have the ball of No, 1 in the midst
of its rays, while the other two remained at the side, unaffected
Here the thermometer No. i rose 7 degrees, in 10 minutes, by
an exposure to the full red coloured rays.
I drew back the stand, till the centre of the ball of No. 1 was
of the invisible Rays of the Sun. 28^
just at the vanishing of the red colour, so that half its ball was
within, and half without, the visible rays of the sun.
No. !• No. 2. No. 3*
481. - - 481 - - 48
55i - - 481- - - 48
57 - - 49 - - 48i
Here the thermometer No. 1 rose 8 degrees, in 10 minutes.
By way of not losing time, in order to connect these last
observations the better together, I did not bring back the ther-
mometer No. 1 to the temperature of the room, being already
well acquainted with its rate of shewing, compared to that of
No. 2, but went on to the next experiment, by withdrawing the
stand, till the whole ball of No. 1 was completely out of the sun's
visible rays, yet so as to bring the termination of the line of the
red colour as near the outside of the ball as could be, without
Here the thermometer No. 1 rose, in 10 minutes, another
degree higher than in its former situation it could be brought
up to ; and was now 9 degrees above the standard. The ball
of this thermometer, as has been noticed, is exactly half an inch
in diameter ; and its centre therefore was ^ inch beyond the
visible illumination, to which no part of it was exposed.
It would not have been proper to compare these last obser-
vations with those taken at an earlier period this morning, in
^90 Dr. llERSCEmfs Experiments on the Ref Tangibility
order to obtain a true maximum, as the sun was now more
powerful than it had been at that time; for which reason, I
caused the hne of termination of visible light, now to fall again
just i inch from the centre of the ball; and had the following
No. 1. No. 2. No. g:
571 - - 50 — - 49f
^i - - 50 - - 49i
5H - - 50 - - 49-
And here the thermometer No. 1 rose, in 16 minutes, 8|
degrees, when its centre was |- inch out of the visible rays of
the sun. Now, as before we had a rising of 9 degrees, and
here 8|^, the difference is almost too trifling to suppose, that
this latter situation of the thermometer was much beyond the
maximum of the heating power; while, at the same time, the
experiment sufficiently indicates, that the place inquired after
need not be looked for at a greater distance.
It will now be easy to draw the result of these observations
into a very narrow compass.
The first four experiments prove, that there are rays coming
from the sun, which are less refrangible than any of those that
affect the sight. They are invested with a high power of heating
bodies, but with none of illuminating objects ; and this explains
the reason why they have hitherto escaped unnoticed.
My present intention is, not to assign the angle of the least
refrangibility belonging to these rays, for which purpose more
accurate, repeated, and extended experiments are required. But,
at the distance of 52 inches from the prism> there was still a
considerable heating power exerted by our invisible rays, one
of the invisible Kays of the Sun. %gt
inch and a half beyond the red ones, measured upon their pro-
jeetion on: a horizontal plane. I have no doubt but that their
efficaGyma^r be traced still somewhat farther.
The 5th and 6th experiments shew, that the power of heating
is extended to the utmost limits of the visible violet rays, but
not bey ond them ; and that it is gradually impaired, as the rays
grow more refrangible. \
The four last experiments prove, that the maximum of the
heating power is vested among the invisible rays ; and is pro-
bably not less than half an inch beyond the last visible ones,
when projected in the manner before mentioned. The same
experiments also shew, that the sun's invisible rays, in their less
refr^igible state, and considerably beyond the maximum, stilt
exert a heating power fuliy equal to that of red^coloured light;
and that^ consequently, if we may infer the quantity of tihe effi'-
cient ftenii the efesti produjced,: the invisible rays of the sun
p^olmbly far exceedr the visible ones in number.
To concMde, if we calli %ii^ those rays wWch illuminate
objects, and radiant heat, those whicli fieat bodies, it may be
inquired, whether light be essentially different from radiant
heat ? In answer to which I would suggest, that w^e are not
allowed, by the rules of philosophizing, to admit two different
causes to explain certain effects, if they may be accounted for
by one. A beam of radiant heat, emanating from the sun,
consists of rays that ate differently refrangible. The range of
their extent, when dispersed by a prism, begins at violtet-
coloured light, where they are most refracted^ and have the
least eflScacy. We have traced these calorifie rays throughout
the whole extent of the prismatic spectrum; and found their
power increasing, while tibeir r^rangibility was^ lessened^ as far
^^2 Dr. Her'schel*s Experiments^ &c.
as to the confines of red-coloured light. But their diminishing
refrangibility, and increasing power, did not stop here ; for we
have pursued them a considerable way beyond the^rtjmaif/f:
spectrum^ into an invisible state, still exerting their increasing
energy, with a decrease of refrangibihty up to the maximum of
their power; and have also traced them to that state where,
though still less refracted, their energy, on account, we may
suppose, of their now faihng density, decreased pretty fast;
after which, the invisible thermometrical spectrum, ii I may so
call it, soon vanished.
If this be a true account of solar heat, for the support of
which I appeal to my experiments, it remains only for us to
admit, that such of the rays of the sun as have the refrangibility
of those which are contained in the prismatic spectrum, by the
construction of the organs of sight, are admitted, under the
appearance of light and colours ; and that the rest, being stopped
in the coats and humours of the eye, act upon them, as they
are known to do upon all the other parts of our body, by occa-
sioning a sensation of heat*
Slough, near Windsor,
March 17, 1800^
Explanation or Plate XL
IN WHICH IS GIVEN A VIEW OF THE APPARATUS.
A B. The small stand.
1, 2, 3. The thermometers upon it.
C D. The prism at the window.
E. The spectrum thrown upon the table, so as to bring the
last quarter of an inch of the red colour upon the stand.
Qt^WJ-:jLru/M . :*a u ^K K::.J7a/<^Xl./?y 2^2.